1. Field of the Invention
The invention relates to gain control or the quantization method for input signals of analog-to-digital conversion or digital-to-digital conversion circuits or process.
2. Description of the Related Art
Analog-to-digital converters (ADC) are circuit components for converting analog signals to digital signals. Analog processing circuits for processing analog signals have complicated circuit structures and therefore higher manufacturing costs. On the contrary, digital processing circuits, such as widely used microprocessors and digital signal processors, can perform complicated signal processing with lower circuit cost. Digital processing circuits are therefore more popular than analog processing circuits, and analog input signals of electronic apparatus are converted into digital signals to be processed by digital processing circuits. Thus, analog-to-digital converters are requisite circuit components of electronic apparatus.
A working range of an analog-to-digital converter is the convertible input voltage variation range thereof. If the voltage of an analog input signal exceeds the working range of an analog-to-digital converter, the digital output signal may suffer from a waveform distortion, or signal-to-noise ratio (SNR) degradation. Because the voltage variation range of an analog input signal does not always coincide with the working range of an analog-to-digital converter, the amplitude of the analog input signal must be carefully adjusted before the analog input signal is fed into the analog-to-digital converter. Thus, an automatic gain control (AGC) module is added to determine a gain of the analog input signal, and an amplifier is used to amplify the analog input signal according to the gain to obtain an amplified analog signal with a voltage variation range coinciding with the working range of the analog-to-digital converter. After the analog-to-digital converter converts the amplified analog signal to a digital signal, the digital signal therefore has a higher signal-to-noise ratio.
Referring to FIG. 1, a flowchart of a conventional gain control method 100 for an analog input signal of an analog-to-digital converter is shown. A conventional gain control module determines the gain for amplifying an analog input signal of an analog-to-digital converter according to the method 100. First, an amplifier amplifies an analog signal according to a gain determined by the gain control module to obtain an amplified analog signal (step 102). The amplified analog signal is then fed to an analog-to-digital converter, and the analog-to-digital converter converts the amplified analog signal from analog to digital to obtain a quantized signal (step 104). The gain control module then estimates the amplitude or power of the quantized signal and compares the value with a reference value (step 106). In some embodiment, the gain control schemes are based on PAPR (peak to average power ratio), RSSI (received signal strength indicator) or the signal amplitude distribution. The gain control module then determines whether the quantized signal amplitude is within the reference range (step 108). If so, the gain for amplifying the analog signal requires no adjustment. Otherwise, the gain control module adjusts the gain for amplifying the analog signal (step 110).
In the design of automatic gain control algorithm, the basic performance criterion of an AGC algorithm is to minimize the quantization noise of the signal after analog-to-digital conversion:
                                          N            Q                    =                                    ∫                              -                ∞                            ∞                        ⁢                                                            (                                      x                    -                                          x                      Q                                                        )                                2                            ⁢                              p                ⁡                                  (                  x                  )                                            ⁢                              ⅆ                x                                                    ;                            (        1        )            wherein NQ is the quantization noise power, x is the amplified analog signal, xQ is the digital signal, and p(x) is the hypothetic signal distribution. The signal distribution p(x) indicates a probability of the amplitude of the amplified analog signal. For example, the hypothetic input signal distribution of a GNSS system is a Gaussian distribution.
When a signal is transmitted from a transmitter to a receiver in a communication system, the signal suffers from interference and noise in the transmission path between the transmitter and the receiver. The receiver therefore receives an input signal comprising interference and noise. Referring to FIG. 2, a schematic diagram of a signal comprising interference and noise is shown. The interference components of the received signal are referred to as jamming components. The jamming component greatly changes the voltage variation range of the received signal and alters the signal distribution of the received signal. As shown in FIG. 2, a sinusoidal wave signal component oscillating between −6 volts and +6 volts is the jamming component.
Because a signal distribution of the jamming component cannot be predicted in advance, the actual jamming environments may vary a lot with different interference types or power, which can not be modeled. If the AGC behavior mode is not properly designed for the changing environment, the AGC gain control is not optimized. The analog input signal property and CN0 are seriously degraded at the ADC quantization stage. The digital signal output by the analog-to-digital converter therefore has low signal-to-noise ratio, degrading system performance. For example, the signal-to-noise ratio may be lower than that corresponding to an optimal gain by 10˜15 dB. Thus, a method for determining a gain for amplifying an analog input signal of an analog-to-digital converter under all signal input environment to increase a signal-to-noise ratio of a digital output signal is therefore required.